The steering gear ratio can be evaluated by the following equation: steering gear ratio=(steering rotational angle)/(wheel rotational angle).
Generally, a vehicle steering system transmits the rotation of a steering wheel to a gear box via a steering shaft. The rotational movement of the steering wheel is converted into a linear movement of a rack and pinion in the gear box to rotate the wheel. Therefore, the rotational angle of the wheel is constantly formed in relation to the rotational angle of the steering wheel. That is, the steering gear ratio is constant in the conventional system.